One responded to the Kennedy-era call for a new generation of leaders and the other renewed that call for a new generation to lead this country in a new direction in a new century. One is an untraditional figure who personifies the slow, traditional path to leadership in American life and the other is an untraditional political figure who wants to leap ahead in the race for American leadership. One is an old-fashioned liberal, the other a new-era conservative.

In many ways the two candidates who entered the presidential race in recent days define the 2016 political contest — and, in important ways, define important struggles that have been part of American life for more than two centuries.

Hillary Rodham Clinton and Marco Rubio stand as more than candidates for the White House. They stand as symbols of the pathways to power in American life.

On the surface Ms. Clinton is a dramatic departure from the ordinary. A woman, a onetime first lady, she stands as a potentially historic figure. But besides her gender and her status as the spouse of a president, her journey, which includes two successful candidacies for the Senate and four years as secretary of state, has more in common with Henry Clay and Daniel Webster — think about this before you fill my email in-box with invective — than with many lesser names who have been elected president.

Perhaps the Clinton skeptics prefer a comparison with Herbert Hoover, probably the only figure in modern history besides Dwight Eisenhower, and perhaps George H.W. Bush, with pre-presidential experience commensurate with Ms. Clinton’s. Either way, she begins her second campaign for the White House with vastly more relevant experience than her husband, who served two terms, had when he first ran in 1991.

At the same time, Mr. Rubio, who also could make history as the nation’s first Hispanic president, conforms to the in-a-hurry pattern that describes all three of the sitting senators — Warren G. Harding, John F. Kennedy and Barack Obama — who ascended to the presidency. Indeed, in many ways Mr. Rubio, a Republican from Florida, sounds more like Mr. Kennedy than does JFK’s fellow Democrat, Ms. Clinton — who, in fact, supported Republican Barry Goldwater in the election a year after Mr. Kennedy’s death. Listen to the Rubio rhetoric:

“While our people and economy are pushing the boundaries of the 21st century, too many of our leaders and their ideas are stuck in the 20th century. We must change the decisions we are making by changing the people who are making them.” He might as well have added, “Let’s get this country moving again.” Or “The torch has been passed to a new generation.”

The two campaigns launched last week began with a whopper (Mr. Rubio’s opening assertion that his decision to run came after “months of deliberation and prayer about the future of our country,” a cringe-worthy remark that strains credulity) and a whimper (Ms. Clinton’s bizarre video, which might have led you to think you were watching an advertisement for a hospital chain or a life-insurance company). In that regard, Ms. Clinton seemed to have channeled Ronald Reagan.

Mr. Rubio’s campaign enters a crowded field, Ms. Clinton’s an empty one — and the difference helps shape the candidates’ personae in the early days of the race.

Mr. Rubio must distinguish himself from Sen. Ted Cruz of Texas and Sen. Rand Paul of Kentucky, who already have entered the race and who share some of the conservative traits (and Tea leaves) he possesses. Of the three, his is the most overtly generational appeal, and he may have the most personal appeal — though his challenge to his onetime mentor, former Gov. Jeb Bush of Florida, wipes away some of that “nice young man” aspect that Mr. Kennedy possessed even after three House and two Senate races.

Mr. Paul, Mr. Cruz and Mr. Bush are only three of the contenders Mr. Rubio must defeat. Some 18 potential Republican presidential candidates (plus Donald Trump) are listed as speakers at this weekend’s First in the Nation Leadership Summit in New Hampshire. Lucky break for Mr. Rubio: He’s the keynoter.

Ms. Clinton has a different challenge entirely. She doesn’t need to distinguish herself from anyone but may need to separate herself from the fin-de-siecle image she possesses. In his announcement remarks, Mr. Rubio spoke of the Democratic front-runner as “a leader from yesterday” who would “take us back to yesterday.”

Ms. Clinton’s yesterdays include the two terms of her husband, with all their tumult but also with all their triumph, and the 2008 campaign, which she entered as the clear favorite only to be eclipsed by Barack Obama and to be hindered by a top-heavy staff that bickered as her prospects withered. (The Clinton presidency began almost a quarter-century ago. It is about the same distance from today as the election of Richard M. Nixon was from Mr. Clinton’s victory over President George H.W. Bush.)

So while Mr. Rubio must struggle for air, and air time, Ms. Clinton has no such struggle. Indeed, when she slipped, incognito, into a suburban Toledo Chipotle outlet, she was served perhaps the most famous chicken burrito bowl (with a side of guacamole) in American history.

The Ready-for-Hillary analysis: Sometimes a presidential candidate just wants a burrito and a blackberry Izze soda. The Hillary-hater analysis: She went quietly into a Chipotle for the express purpose of having the world think she went quietly into a Chipotle, a phony gambit to suggest she’s just a regular gal on a regular road trip … to Iowa.

But there’s nothing regular about her campaign, nor about the reaction to it. The Republicans’ response has been, to resurrect a phrase from Hunter S. Thompson, a combination of fear (she defeats every GOP candidate in head-to-head early polling) and loathing (the Republican National Committee is undertaking a #StopHillary campaign). Mr. Rubio has the disadvantage of not having that disadvantage.

Mr. Rubio knows there has been a number of first-chance presidential triumphs, some even more improbable than his own (Woodrow Wilson, Jimmy Carter). And Ms. Clinton knows that second-chance candidacies can be successful (Mr. Nixon and Mr. Reagan, though by some counts Mr. Reagan’s main chance came on his third chance).

But for both candidates, the goal is clear. They must make voting for them seem like making a choice, not taking a chance. This month the campaign, and that challenge, came into focus.

David Shribman, a Pulitzer Prize winner in journalism, is executive editor of the Pittsburgh Post-Gazette.